Electrically actuated detonators comprising a thermo-resistive element coated with an initiating "flashing" pyrotechnic compound are well known. Such devices are inexpensive to manufacture and are known to be extremely reliable in use as well as relatively safe in storage and handling.
Electrical detonators have a major disadvantage in that they require connection via electrically conductive cables to a remote detonating device providing a source of electrical energy. Accidental explosions have been attributed to spurious electrical currents induced or conducted in the electrical conductors by electrical machinery, static earth charges, lightning, high voltage transmission line corona discharge, radio frequency transmission and the like.
In an endeavour to overcome the dangers associated with conventional electrically actuated detonators of the type described above, various non-electric systems have been proposed.
One type of non-electrically actuated detonator described in the known patent specifications relating to blasting detonators systems employs a tubular lead, the inner surface of which is coated with a deflagrating material. When the deflagrating material is fired at a remote end of the tubular lead, a shock wave is propagated down the interior of the tubular lead to detonate the explosive composite of the detonator. A non-electrical blasting initiating system of this type is described in U.S. Pat. No. 4,757,764.
While generally effective for its purpose and relatively safe in use such non-electric initiating systems have been the cause of a number of large blast project failures and are limited to a short shelf life. The system is complicated to systemize with a shock wave speed of 1.8 kilometers per second compared to 300,000 kilometers per second for optic initiation.
Swedish patent application number 8,503,595 describes a fiber optic ignition system for explosive compositions wherein optical energy is converted by a photo-voltaic cell into electrical energy. The electrical energy is amplified and stored in a capacitor for selective release via electrical conductors to a conventional electric detonator. While this system avoids the necessity for long electrical conductors between the detonator and the remote initiating position (with the attendant risks described above) the requirement of expensive fiber-optic cables capable of transmitting a large amount of energy to charge the capacitor, with its ability to hold the full charge for only a short period. The firing of the detonator would require a further signal from the remote triggering station.
The formation of a continuous electrically conductive circuit between the electrical switching means of the optical initiating device and the detonator is considered to pose an unacceptable risk of premature detonation from spurious induced or conductive currents in the electrical conductors.
U.S. Pat. Nos. 3812783,4403143 and 3408937 are illustrative of non-electric blast initiation systems employing high intensity laser radiation transmitted via an optical fire cable to a detonating device.
Prior art blast initiating systems employing Laser energy as a sole energizing source are relatively safe in use but currently uneconomical in use and of dubious reliability.
It is an aim of the present invention to overcome or alleviate the problems of prior art blast initiation systems and to provide a safe, reliable and economic explosive detonator and initiation system therefore.